Natural gas is a byproduct formed during oil extraction from oil wells, and is typically referred to as wellhead gas. Wellhead gas comprises a mixture of methane, ethane, propane, nitrogen, carbon dioxide, and water. In addition, wellhead gas may contain varying amounts of sulfur compounds such as hydrogen sulfide. Because of the remote location of the well sites, it is often not economical to collect this gas and transport it for further value added processing. Rather, the gas is flared, a term that refers to the combustion of wellhead gases.
Flaring or combustion of wellhead gases is initiated and controlled using a burner management system. A burner management system is also used to control ignition in chemical process burners and incinerators. The burner management system controls the operation of an igniter. Ignition in turn could be achieved by spark ignition or sparkless ignition. Further, depending on the quantity of gas produced at the wellheads, or the flow rate of fuel in process burners, or the heat duty of the ignition system, the system could use a pilot flame, or could be pilotless. Generally, ignition systems rated at >125,000 BTU utilize a pilot to initiate ignition. The pilot flame is fueled by a dedicated fuel line and is available to ignite relieved gases when needed. For example, Zeeco Inc.'s wellhead flare is equipped with high stability, low flow spark ignited pilot, which can withstand hurricane force winds of 170 mph with zero flame failure. In the case of spark ignition, the sparking tips require periodic cleaning to remove carbon accumulation formed as a byproduct of combustion. Further, periodic adjustment is required to maintain the spark gap between the two electrodes in a spark igniter. Therefore, there is an increasing interest in using sparkless ignition for piloted systems.
U.S. application Ser. No. 13/284,393 entitled “HOT SURFACE IGNITION ASSEMBLY FOR USE IN PILOTS FOR FLARING, INCINERATION, AND PROCESS BURNERS” filed on Oct. 28, 2011, describes a sparkless ignition device for a piloted flare. A hot surface ignition (HSI) assembly is positioned proximate to the pilot head and produces heat by induction sufficient to ignite the pilot fuel. The HSI operates at or above temperatures of 2100° F. (about 1149° C.). A thermocouple is positioned near the pilot head to sense the temperature of the pilot exhaust gas. A control system uses the measured temperature to reduce, start, or stop the current passing through the insulated element of the HSI. The HSI assembly could be affixed to the pilot head by a threaded or welded fitting. The HSI assembly is built to be a drop-in replacement to sparking technology that is widely used today. Igniters, and methods to enable reliable operation of igniters without the use of a flame rod or a thermocouple to sense a flame were not disclosed.
U.S. application Ser. No. 11/047,794 entitled “METHOD, APPARATUS AND SYSTEM FOR CONTROLLING A GAS-FIRED HEATER,” and filed by the applicant on Feb. 1, 2005, and incorporated by reference herein in its entirety, discloses that the HSI assembly preferably comprises a silicon nitride element that has a rated temperature of at least 1000° C. at 12 volts. However, the above application did not disclose igniters and methods to use the HSI assembly itself to detect the presence or absence of a pilot flame, or how to solve the problem of temperature quenching of the HSI assembly in a compact pilot igniter when the HSI assembly is also used as a flame sensor.
A flame sensor such as a flame rod or a thermocouple is used to detect a flame, and feeds a suitable signal to the burner management system. In the case of a flame rod, an AC current is applied to the flame rod such as Kanthal flame rods rated to 2600° F. (available for example, from Honeywell), which then flows through the ions in the flame, and to the pilot assembly/head to ground. Because the surface area of the flame rod is much smaller than that of the pilot head, the AC current is rectified to DC current in the process commonly known as flame rectification. The magnitude of this current could vary from 0.25 to 8 mA. Armored wiring harness rated at 500° F. or above is used. The burner management systems opens the main gas valve to the igniter if it detects a DC current of pre-determined magnitude. Flame rods require periodic maintenance because of carbon formation (soot) on the sensors. In addition, the extreme temperatures seen at remote wellheads may also lead to crack formation in the ceramic insulators of the sensor rods. Finally, the sensor rods also tend to corrode.
As an alternative to flame rods, thermocouples may be used to sense the temperature of the flame and/or exhaust gases. An exemplary thermocouple is the K-type thermocouple, which is rated to 2400° F. Because these thermocouples are located in the flame, they are usually sheathed in high temperature metal sheaths such as Inconel. Inconel sheaths increase the cost of thermocouples, and thereby the cost of the igniters. Further, they are subject to the deficiencies that are seen in flame rods. Finally, even when the flame goes out, the measured temperature gradually decreases, resulting in a lag period between the time the flame goes out and the time the burner management system senses that the flame is indeed out and takes corrective action. Unburnt fuel is therefore exhausted into the atmosphere. An alternative to flame rods and thermocouples is therefore desired to minimize costs and to improve the operation and reliability of sparkless igniters and ignition systems.
HSI assemblies can also be used in pilotless burner systems. U.S. Pat. No. 8,434,292 entitled “CERAMIC-ENCASED HOT SURFACE IGNITER SYSTEM FOR JET ENGINES,” and issued on May 7, 2013, discloses HSI assemblies for igniting fuel in jet engines. The disclosed HSI assemblies were encased in silicon nitride. These fit-for-purpose specially designed HSI assemblies were capable of maintaining temperature in turbulent flow conditions commonly seen in an operating jet engine, and employed specific geometries of the ceramic encasement, multiple igniter elements, and control strategies. Since the igniter temperature maybe quenched due to convective cooling, the voltage to the igniter element could be increased by the control system, thereby increasing the power flowing through the internal resistive heating element. This action created a corresponding rise in temperature to allow the HSI assembly's external surface to reach temperatures sufficient for auto ignition to occur. A sparkless igniter for use as a pilot that employs commercially available HSI assemblies and designs to mitigate the effects of convective cooling was however not disclosed.
The applicant currently sells sparkless igniters for pilot services. An example of a sparkless pilot igniter 400 is shown in FIG. 1(a). Pre-mixed fuel and air enters the igniter 400 and flows into the nozzle 401. The nozzle is removably connected to neck 403 of the igniter. The igniter is equipped with a flame rod 402, which senses the presence or absence of the pilot flame, and feeds the signal to a burner management system. In some models, a thermocouple is used instead of the flame rod. FIG. 1(b) provides additional details related to the location of a hot surface igniter (HSI) assembly 405 inside the igniter. HSI assembly 405 is cylindrical in shape and comprises a heating element 406 that is substantially enclosed in a high temperature ceramic body 407. A high temperature alloy guard (e.g. Inconel guard) 409 protects the ceramic body, and the exposed part of the heating element 406. The HSI assembly is positioned such that the tip 408 is located upstream of the nozzle throat 404. When the energized hot element 406 is exposed to a fuel air mixture, a flame is produced and extends through the throat of the nozzle 404 and into the nozzle 401. The flame body (or plume) therefore sits above the tip 408 and the nozzle throat 404. Since the tip does not sit in the flame plume, it is subject to convectional cooling by the flow of the incoming fuel-air mixture. As a result, the HSI assembly in this arrangement cannot be used to detect the presence or absence of the pilot flame reliably by measuring the change in resistance of the element 406 prior to, and after ignition. This is because the resistance of element 406 is a function of temperature, and convectional cooling of the element 406 would lead to an artificial change in temperature (and hence resistance) that is not related to the presence or absence of a flame. A flame rod 402 (or thermocouple) is therefore required to sense the flame and adds on to the cost of the igniter, in addition to requiring maintenance, as described above.
FIG. 1(c) schematically shows the installation of the sparkless pilot igniter 400 in the burner management system. At start-up, the HSI element in the pilot igniter is energized before the fuel is fed to the pilot. The fuel pressure is controlled to about 5 psig using regulator PRV-1001, and the fuel is routed through a fail-safe pilot fuel valve SV-1002 to the sparkles pilot igniter. Auto-ignition of the fuel-air mixture on the hot surface of the HSI element of the pilot igniter is detected using a flame sensor (thermocouple or flame rod). If a flame is detected, fuel is routed through main fuel burner valve MV-2002 to the main burner. If a flame is not detected at the pilot within the predetermined time, the fuel valves SV-1002 and MV-2002 close and the system recycles.
U.S. Pat. No. 4,405,299 entitled “BURNER IGNITION AND FLAME MONITORING SYSTEM” and issued on Sep. 20, 1983, discloses using a hot surface ignitor as both an ignition element and as a flame rectification sensor (flame rod). A control system alternates between an ignition control circuit and a flame sensing circuit using an ignition control switch. This method requires the use of an alternating current source, which is available in residential homes, but not at remote well sites. Alternative methods and devices for using a DC input source to energize hot surface igniters to ignite a fuel at remote sites, and to utilize the resistance of the igniter element to detect the presence or absence of flame are therefore desired.
A compact, sparkless pilot igniter that can reliably operate without the use of thermocouples or flame rods to sense a flame in conjunction with a suitable burner management system is therefore desired.